Advance and consideration on the mechanism of formation and emplacement of granitic magma

Granite (sensu lato) that belongs to the continental crust is the rock of distinguishing the lithosphere of the earth from that of other planets. Geological evidence from both planetary exploration and ODP constrain the formation of granite: no granite was generated during transition from magmatic surface to rocky shell of the inner planets, and formation of granite and its related continental crust should initiate after the occurrence of sedimentary rocks on the earth' s surface. The average growth rate of granite in 2-D space of the lithosphere is about 485 X 10 km/Myr and magma is considered mainly from the partial melting of crustal rocks (anatexis). On the basis, we introduce the progression in the study of crustal anatexis and the relationship between rheological behaviors and melt fraction of the partially melted rocks, and compare the similarities and differences of the both models, i. e. magma intrusion and the magma convection, on explaining the formation-emplacement mechanism of granitic magma. The magma source and its related granite body are separated in the magma intrusion model. One of the difficulties for the model is that the magma source is located beneath its related granite body and thus used to be unobservable unless the granite body and the rocks between the granite and the source have been moved out by erosion or structure. Finally, we brief the study advance of intra-crustal magma convection. In the convection model, the source and the emplacement place of magma are regarded as a whole. When the melt fraction of the rocks in the source region reaches the solid-liquid transition (SLT), the rocks change into dirty magma. Gravitational differentiation within the dirty magma layer initiates heat convection that results in moving up of the MI (SLT) and thickening of the crustal magma layer. It is concluded that thermal convection within a crustal melting layer is essential for formation of granite magma; without convection, partial melting generates migmatite, but not magma that forms granite batholiths.